1. Field of the Invention
The present invention relates to optical filters and, in particular, to multi-window wavelength filters using fused-fiber unbalanced Michelson Interferometers.
2. Discussion of the Related Art
With existing fiber optic networks, there is often the need to increase information transmission capacity. However, both physical and economic constraints can limit the feasibility of increasing transmission capacity. For example, installing additional fiber optic cable to support additional signal channels can be cost prohibitive, and electronic system components may impose physical limitations on the speed of information that can be transmitted. One way to increase the capacity of an existing fiber optic link without modification to the fiber itself is by multiplexing multiple signals via wavelength division multiplexers (WDMs). The use of WDMs provides a simple and economical way to increase the transmission capacity of fiber optic communication systems by allowing multiple wavelengths to be transmitted and received over a single optical fiber through signal wavelength multiplexing and demultiplexing. The demultiplexed signals can then be routed to the final destinations. WDMs can be used in fiber optic communication systems for other purposes as well, such as dispersion compensation, noise reduction, and gain flatting, i.e., maintaining a uniform gain within the usable bandwidth for erbium-doped amplifiers.
WDMs can be manufactured using, for example, biconical tapered fusion (BTF) technology. Typically, two optical fibers are fused together along an interior portion to form a fused-fiber coupler, so that light of two wavelengths (i.e., 1310 nm and 1550 nm) entering the input terminals of the first and second fibers, respectively, are multiplexed onto a single fiber. The coupling ratios for the two channels (the signals at 1310 nm and 1550 nm) exhibit complementary sinusoidal behavior for amplitude as a function of frequency within the passband of the WDM, with each channel having one or more peaks (or windows) within the passband. Information carried by the two signals along the single fiber is then demultiplexed at the WDM outputs.
Multi-window WDMs (MWDMS) have two or more peaks of amplitude as a function of frequency (or operational windows) for each channel within a passband. MWDMs can also be made using BTF technology by twisting two optical fibers together, fusing the center portion together, and pulling the fibers until a desired multi-window transmission spectrum appears at a monitored fiber output terminal. Such a long-tapered-fusing technology is discussed in commonly-owned U.S. Pat. No. 5,809,190, entitled "Apparatus and Method of Making a Fused Dense Wavelength-Division Multiplexer", which is incorporated by reference herein in its entirety.
One essential component of multiplexing and demultiplexing multiple signals is the ability to accurately separate the individual signals of different wavelengths from the combined signal. Many techniques for demultiplexing wavelength-multiplexed signals have been developed and proposed. One conventional method uses prisms and diffraction gratings to spatially separate signals of different wavelengths from a fiber. These devices suffer from lack of programmability and flexibility, poor switching performance, and the difficulty of integration. Moreover, such systems often require precision positioning of various components to ensure desired optical alignment for proper operation.
Other techniques utilize optical bandpass filters that selectively pass an optical beam of a specified wavelength. The optical filter is capable of selecting and separating any desired wavelength out of a wavelength division multiplexed optical signal. In the wavelength multiplex network, a large number of information signals are multiplexed on a single optical fiber, and such a very large number of optical filters, each having its own passband and a central passband wavelength, are needed in order to demultiplex the optical signals or to switch the connection of the optical channels.
The underlying principle behind the operation of some optical filters is multi-beam optical interference. Here, the optical path length of the individual light beams in the device is varied, after which the beams are interfered. Since the effective path length is wavelength dependent, it follows that the interference is wavelength dependent, which yields a wavelength dependent optical response. Each filter can be tuned to pass signals at a desired wavelength, while attenuating or blocking signals at all other wavelengths.
Conventionally, the optical filters for use in the optical telecommunication network have been formed based upon a multi-layer mirror or a Fabry-Perot resonator. In such conventional filters, the passband wavelength has been determined by the construction of the filter such as the geometrical dimension or the composition of the material. Thereby, one has to provide a large number of filters in correspondence to the signal channels of the telecommunication system, together with a distribution network for distributing the optical signals to each of these filters. When the number of the optical channels is increased, such a construction becomes inevitably bulky and expensive. Other typical filters include liquid crystal tunable filters (LCTFs), acousto-optical tunable filters (AOTFs), and reflective waveguide arrays. These and other types of optical filters can also be difficult to tune and/or subject to external influences such as temperature, movement, etc.
In addition, even though optical fibers have high information capacity, the overall optical communication link may be restricted by practical bandwidth considerations, thereby limiting the size of the passbands. Therefore, to increase the efficiency of bandwidth use, the passband should contain as many communication channels or windows as possible, subject to constraints with the system. To increase the number of channels or windows, the bandwidth of each channel or window and/or the separation between wavelength peaks should be minimized. Separating signals passed through MWDMs can be accomplished with tunable multi-window wavelength filters. However, as the number of channels within a passband increases, it becomes more difficult for the multi-window wavelength filter to separate or filter the various wavelengths.
Accordingly, small-sized multi-window wavelength filters which are durable and tunable to desired wavelengths are desired.